During the fabrication of many micro-electro-mechanical (MEMS) devices it is required to etch a layer of material to completion stopping on the layer below (e.g., Silicon on Insulator (SOI)—clearing a silicon (Si) layer stopping on an underlying silicon dioxide (SiO2) layer). Allowing the etch process to proceed beyond the time when the first layer has been removed can result in reduced thickness of the underlying stop layer, or feature profile degradation (known in the art as “notching” for SOI applications).
As a result, it is crucial in a plasma processing process such as etching that the endpoint of the plasma processing be judged accurately to end the plasma processing with no delay. As a method for detecting the endpoint of plasma processing, a method in which any change in the light spectrum of a specific substance contained within the plasma in the processing chamber is detected, with the endpoint being detected based upon such change, is well known in the art. This method, which is conceived from the observation that the contents in the plasma change as the etching on the substrate progresses, aims to detect a real-time endpoint of the etching process accurately by monitoring a change in the intensity of the light spectrum of a specific substance. This method commonly used to detect plasma process termination times is optical emission spectrometry (OES).
OES analyzes the light emitted from a plasma source to draw inferences about the chemical and physical state of the plasma process. In semiconductor processing this technique is commonly used to detect material interfaces during plasma etch processes. The OES technique involves monitoring the radiation emitted by the plasma, usually in the ultra violet/visible range (200 nm–1100 nm) portion of the light spectrum. FIG. 1 shows a schematic view of a typical OES configuration. The composition of the plasma, and in particular the presence of reactive etch species or etch by-products, will determine the spectra (i.e., intensity vs. wavelength) of the emitted radiation. During the course of an etch process, and especially at a material transition, the composition of the plasma changes, resulting in a change in the emission spectrum. By continuously monitoring the plasma emission, it is possible for an OES endpoint system to detect that change and use it to determine when the film has completely cleared. For example, when the OES signal drops below a pre-determined threshold level, this transition is used to trigger “endpoint”. In practice, most of the information relating to endpoint is usually contained within a few wavelengths that correspond to reactants consumed or the etch by-products that are generated during the etch.
A common method to develop an OES endpoint strategy is to collect a number of spectra of the plasma emission (emission intensity v. wavelength) during both pre-endpoint and post-endpoint conditions. Endpoint wavelength candidate regions can be determined using a number of methods. Spectral regions for endpoint detection can be chosen through statistical methods such as factor analysis or principal component analysis (see U.S. Pat. No. 5,658,423 to Angell et al.). Another strategy to determine endpoint candidates is through the construction of a difference plot between pre-endpoint (main etch) and post-endpoint (over etch) spectra. Once candidate regions have been selected, assignments of likely chemical species are made for the candidate regions (i.e., reactant species from dissociated gas precursors or etch products). The assignment is not critical in determining success of the strategy, but rather assists in understanding and optimizing the wavelength selection process. A number of references including Tables of Spectral Lines by Zaidel et al. and The Identification of Molecular Spectra by Pearse et al. in conjunction with knowledge of the process chemistry can be used to assign likely species identities for the candidate lines. An example of likely endpoint candidates for a silicon etch. process in a sulfur hexafluoride (SF6) plasma would be fluorine (F) lines at 687 nm and 703 nm as well as the silicon fluoride (SiF) emission band at 440 nm. Once these regions have been determined, subsequent parts can be processed using the same OES strategy.
While these OES approaches work well for single step processes or processes with a limited number of discrete etch steps (such as an etch initiation followed by a main etch), it is difficult to apply OES to plasma processes with rapid and periodic plasma perturbations. Examples of such time division multiplexed (TDM) processes are disclosed in U.S. Pat. No. 5,501,893 to Laermer et al., U.S. Pat. No. 4,985,114 to Okudaira et al., and U.S. Pat. No. 4,795,529 to Kawasaki et al. Laermer et al. disclose a TDM process for etching high aspect ratio features into Si using an alternating series of etch and deposition steps.
FIGS. 2(a) to 2(d) are pictorial examples of one type of the TDM process for deep silicon etching. The TDM Si etch process is typically carried out in a reactor configured with a high-density plasma source, typically an Inductively Coupled Plasma (ICP), in conjunction with a radio frequency (RF) biased substrate electrode. The most common process gases used in the TDM etch process for Si are sulfur hexafluoride (SF6) and octofluorocyclobutane (C4F8). SF6 is typically used as the etch gas and C4F8 as the deposition gas. During the etch step, SF6 facilitates spontaneous and isotropic etching of Si (FIGS. 2(a) and 2(b)); in the deposition step, C4F8 facilitates protective polymer deposition onto the sidewalls as well as the bottom of etched structures (FIG. 2(c)). The TDM Si etch process cyclically alternates between etch and deposition process steps enabling high aspect ratio structures to be defined into a masked Si substrate. Upon energetic and directional ion bombardment to the Si substrate, which is present in etch steps, the polymer film coated in the bottom of etched structures from the previous deposition step will be removed to expose the Si surface for further etching (FIG. 2(d)). The polymer film on the sidewall of the etched structures will remain because it is not subjected to direct ion bombardment, inhibiting lateral etching. Using the TDM Si etch approach allows high aspect ratio features to be defined into Si substrates at high etch rates. FIG. 2(e) shows a scanning electron microscope (SEM) image of a cross section of a silicon structure etched using a TDM process.
As shown in FIG. 3, the plasma emission spectra of etch 300 and deposition 305 steps in a TDM Si etch process are dramatically different due to the different plasma conditions that exist in the deposition and etch steps (e.g., process gas types, pressures, RF powers, etc.). As shown in FIG. 4, applying conventional OES methods to a TDM silicon etch process results in an end point trace 400 that is periodic, and cannot be used to detect endpoint. For the TDM Si etch, it is expected that the majority of the etch endpoint information is contained within the etch segments of the process.
U.S. Pat. No. 6,200,822 to Becker et al. shows a method to extract endpoint information from the plasma emission of a TDM Si etch process. Becker et al. examine the emission intensity of at least one species (typically F or SiF for an Si etch) in the plasma only during the etch step through the use of an externally supplied trigger (typically the transition from one process step to the next). By using an external trigger in conjunction with a delay function and a sample-and-hold (peak-hold) circuit, the emission intensity observed in subsequent etch steps can be stitched together to obtain an emission signal that is not periodic in nature. The value of the emission intensity for the species in the etch step is held at the last known value during the ensuing deposition step. In this manner the periodic emission signal is converted into a curve similar to a step function that can be used for process endpoint determination. The limitations of this approach are the need for an externally supplied trigger, in addition to the need for a user input delay between the trigger and acquiring the emission data during etch steps.
In an effort to increase the OES method sensitivity U.S. Pat. No. 4,491,499 to Jerde et al. disclose measuring a narrow band of the emission spectrum while simultaneously measuring the intensity of a wider background band centered about the narrow band. In this manner the background signal can be subtracted from the endpoint signal resulting in a more accurate value of the narrow band signal.
Therefore, there is a need for an endpoint strategy for TDM plasma processes that does not require an external trigger and a user input delay past the trigger to synchronize the plasma emission data collection with the process steps.
Nothing in the prior art provides the benefits attendant with the present invention.
Therefore, it is an object of the present invention to provide an improvement which overcomes the inadequacies of the prior art devices and which is a significant contribution to the advancement of the semiconductor processing art.
Another object of the present invention is to provide a method for etching a feature in a substrate comprising the steps of: subjecting the substrate to an alternating process within a plasma chamber; monitoring a variation in plasma emission intensity; extracting an amplitude information from said plasma emission intensity using an envelope follower algorithm; and discontinuing said alternating process at a time based on said monitoring step.
Yet another object of the present invention is to provide a method of establishing endpoint during a time division multiplex process comprising the steps of: subjecting a substrate to the time division multiplex process; monitoring an attribute of a signal generated from the time division multiplex process; processing said attribute of the periodic signal generated from the time division multiplex process using an envelope follower; and discontinuing the time division multiplex process at a time based on the processing step.
Still yet another object of the present invention is to provide a method for establishing endpoint during a time division multiplexed process, the method comprising the steps of: etching a surface of a substrate in an etching step by contact with a reactive etching gas to removed material from the surface of the substrate and provide exposed surfaces; passivating the surface of the substrate in a passivating step during which the surfaces that were exposed in the preceding etching step are covered by a passivation layer thereby forming a temporary etching stop; alternatingly repeating the etching step and the passivating step; analyzing an intensity of at least one wavelength region of a plasma emission through the use of an envelope follower algorithm; and discontinuing the time division multiplexed process at a time which is dependent on said analysis step.
Another object of the present invention is to provide a method of establishing endpoint during a time division multiplex process comprising the steps of: subjecting a substrate to the time division multiplex process; monitoring an attribute of a signal generated from the time division multiplex process; processing said attribute of the periodic signal generated from the time division multiplex process using a peak-hold and decay algorithm; and discontinuing the time division multiplex process at a time based on the processing step.
The foregoing has outlined some of the pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.